Trenching apparatus

ABSTRACT

A pair of rotatable cutter heads are supported from the carriage for disposal at least partially beneath the pipeline as the carriage moves forwardly along the pipeline, and cuttings dug by the cutter heads are removed from the trench through suction conduits.

United States Patent 11 1 Bates, Jr. et al. 14 1 Feb. 20, 1973 54 TRENCHING APPARATUS 3,429,132 2/1969 Martin ..6l/72.4 3,338,059 8/1967 Tittle ..6l/72.4 [75] Inventors i g Jack 3,462,963 8/1969 Moore... ..61/72.4 3,103,770 9/1963 Popich ....61/72.4 [73] Assignee: 0ceanoics,1nc., Houston, Tex. 3,004,392 10/1961 Symmank 61/72.4

F- 2 7 [22] Oct 19 0 Primary Examiner-Jacob Shapiro PP N04 83,806 Att0mey-Hyer, Eickenroht, Thompson & Turner 52 US. Cl ..61/72.4, 37/63 1 1 ABSTRACT [51] Int. Cl. ..E02f 5/02, F161 1/00 A pair of rotatable cutter heads are supported from [58] Fleid of Search ..61/72.4, 72.1; 37/63 the carriage for disposal at least partially beneath the pipeline as the carriage moves forwardly along the [56] References Cited pipeline, and cuttings dug by the cutter heads are UNITED STATES PATENTS removed from the trench through suction conduits.

3,583,170 6/1971 De Vries ..61/72.4 24 Claims, 9 Drawing Figures TRENCHING APPARATUS This invention relates to apparatus for digging a trench beneath a pipeline. More particularly, it relates to improvements in apparatus of the type shown, for example, in U. S. Pat. Nos. 3,429,131 and 3,429,132, wherein one or more rotary cutter heads are supported from a carriage for digging a trench beneath a pipeline over which the carriage is moved, whereby, upon removal of the cuttings from the trench, the trailing portion of the pipeline is free to fall into the trench.

In the apparatus of these prior patents, each cutter head comprises a cage into which a suction conduit extends to receive cuttings from the head. These cuttings are drawn through the conduit for removalfrom the trench by a suction pump remote from the carriage. When the pump is aboard a vessel above water level and the pipeline is submerged, the pump must necessarily be of large capacity and thus relatively expensive.

The carriage of such earlier apparatus is moved along the pipeline by driven wheels which ride along the top of the pipeline. More particularly, the wheels are arranged fore and aft of the'cutt'er heads, so as to distribute load lengthwise of the carriage. However, it has been found that when the pipeline is at least partially buried, as is often the case when submerged, mud tends to accumulate in front of the forward wheel and thereby impede forward movement of the carriage.

Since the prior apparatus is designed for relatively large pipe, it may actually bend smaller pipe, or, in any case, cause it to be further buried in the mud. Although this load on the pipe may be lessened by the use of outriggers slidable along the ground level at each side of the trench, this lessens the traction between the drive wheels and the pipeline. Also, outriggers cause the carriage to read the elevation of the ground level over which they slide, rather than that of the pipeline.

Furthermore, even when used in trenching beneath larger pipelines, the prior apparatus tends to be top heavy and relatively unstable in a lateral direction. Still further, since the carriage wheels are maintained on the pipeline only due to gravity, jarring of the apparatus may cause the wheels to become untracked from the pipeline or otherwise damage parts of the apparatus, such as the cutter heads disposed beneath the pipeline.

An object of this invention is to provide such apparatus in which the suction pumps may be lighter and thus of less expensive construction.

A further object is to provide such apparatus in which soil in which the pipeline may be at least partially buried will be removed from in front of the driving wheels.

Another object is to provide such apparatus in which lateral and vertical movement of the carriage is limited in a way to prevent untracking of the wheels or damage to the cutter heads in the event of jarring.

Still another object is to provide such apparatus which-imposes less load on the pipeline, but in which traction is nevertheless maintained between the driving wheels and the pipeline, and further in which the apparatus is relatively stable from side-to-side as well as fore and aft.

Yet a further object is to provide such apparatus which is of relatively compact and inexpensive construction.

These and other objects are accomplished, in accordance with the illustrated embodiment of the invention, by apparatus of this type in which each of a pair of cutter heads is mounted on an arm connected to the carriage for swinging about a substantially vertical axis between operative cutting positions at least partially beneath the bottom of the pipeline and relatively widely spaced-apart positions for movement vertically past opposite sides of the pipeline during installation and removal of the apparatus. At least one suction conduit is also mounted on each arm for receiving cuttings from the cutter head thereon, and preferably there are a pair of such conduits, one extending into the cage of each cutter head and other extending behind the cage. With the cutter heads and conduits supported from the carriage in this manner, suction pumps may be connected directly to and thus mounted on the conduits without interfering with one another during movement of the heads between their alternate positions. Consequently, the pumps may be of smaller and less expensive construction.

In accordance with another novel aspect of the illustrated embodiment of the present invention, the carriage is supported on the pipeline by means of a wheel assembly which is mounted on the carriage rearwardly of the cutter heads. in this manner, mud or other debris on the pipelineis removed by the cutter heads prior to movement of the wheel assembly along the pipeline. Preferably, the arms on which the cutter heads are mounted swing about axes disposed rearwardly of the cutter heads and at least one wheel of the wheel assembly, whereby the apparatus is not of excessive length.

The load or weight of the apparatus is reduced by the mounting of one or more buoyancy tanks on the carriage, and traction is maintained between the carriage and pipeline by means of a wheel assembly comprising first and second wheels engaging the top and bottom of the pipeline, respectively, with a constant load. The second or lower wheel is connected to the carriage in such a manner that it may be moved from its position engaging the bottom of the pipeline to a position to the side thereof to permit installation and removal of the apparatus.

The wheel assembly preferably includes a pair of wheels for engaging the top of the pipeline fore and aft of the wheel for engaging the bottom of the pipeline. Movement of each of the upper wheels upwardly with respect to the carriage is limited, and the lower wheel is urged upwardly against the bottom of the pipeline by a yieldable force which tightly clamps the between it and the upper wheels.

Movement of the carriage in any direction with respect to the pipeline is limited by at least one set of rollers carried by a frame mounted on the carriage for disposal closely adjacent all four sides of the pipeline. These rollers not only maintain the wheels in engagement with the pipeline, but also cooperate with the wheels in maintaining traction and stability of the carriage even during movement of the wheels over irregular portions of the pipeline.

Preferably, there is a single buoyancy tank of relatively large capacity toward the forward end of the carriage, and a pair of relatively smaller buoyancy tanks on opposite sides of the rear end of the carriage. These tanks are arranged with their centers of buoyancy fore and aft, respectively, of the cutter heads and wheels mounted on the carriage.

pipeline In the drawings, wherein like reference characters are used throughout to designate like parts:

FIG. 1 is an elevational view from one side of apparatus constructed in accordance with the present invention, and during its movement forwardly along a pipeline;

FIG. 2 is a top plan view of a portion of the apparatus, as shown in FIG. 1, and with the buoyancy tanks removed for purposes of clarity;

FIG. 3 is a view similar to FIG. 2, but with the cutter cages moved outwardly to positions in which they may be moved vertically past opposite sides of the pipeline;

FIG. 4 is a vertical sectional view of the apparatus, as seen along broken line 4-4 of FIG. 1;

FIG. 5 is a vertical cross-sectional view of part of the apparatus, as seen along broken line 5-5;

FIG. 6 is a vertical sectional view of the apparatus, as seen along broken line 6-6 of FIG. 1;

FIG. 7 is an elevational view of a portion of the opposite side of the apparatus, as seen along broken line 7-7 of FIG. 6;

FIG. 8 is a front end view of the upper portion of the apparatus; and

FIG. 9 is a rear end of the apparatus.

With reference now to the details of the abovedescribed drawings, the over-all apparatus, which is indicated in its entirety by reference character 20, is shown in FIG. 1 as it moves forwardly (right to left) along a pipeline P resting on ground surface G to dig a trench T therebeneath. As in prior apparatus of this type, and as will be more fully understood to follow, the

apparatus has means for removing cuttings from the trench T so that the trailing portion of the pipeline is free to fall into the trench, after which it maybe covered in any suitable manner. More particularly, this apparatus is especially well suited for digging a trench beneath a pipeline P at an underwater level, and thus, as will also be understood from the description to follow, is operated by a source of power supplied to it from a vessel (not shown) at water level, much in the manner described in connection with the aforementioned U.S. Pat. No. 3,429,132.

As also best shown in FIG. 1, the apparatus 20 includes an elongate frame 21 which is supported by the pipeline P for movement longitudinally forwardly therealong by means of a wheel assembly including front and rear upper wheels 22 and 23 mounted on the carriage 21 for engaging the top of the pipeline and a lower wheel 24 mounted thereon for engaging the bottom of the pipeline. The upper wheels are driven and the lower wheel is urged tightly against the pipeline so that all wheels grip the pipeline with sufficient traction to move the carriage forwardly.

A pair of cutter heads 25 are mounted on opposite sides of the carriage for disposalin closely spacedapart, side-by-side relation, and thus at least partially beneath the pipeline, as the carriage is movedforwardly therealong. More particularly, the cutters comprise frusto-conically shaped, rotatable cages of substantially the same construction shown in the aforementioned U.S. Pat. No. 3,429,132, and are therefore shown in broken lines. As in the prior patent, they are arranged to rotate about a substantially vertical axis so as to dig trench T as they move forwardly with the carriage.

As shown in the drawings, the cutter heads are arranged forwardly of the wheel assembly so as to remove soil from the pipeline in front of the front upper wheel 22. For this purpose, and as best shown by the broken lines in FIGS. 1 and 4, the upper ends of the cutter head are disposed above the top level of the pipeline and provided with recesses for disposal closely adjacent the sides and bottom of the pipeline when the cutter heads are in operative position.

The apparatus also includes two sets 83 of rollers mounted on the carriage for disposal about the pipeline fore and aft of the wheel assembly. As will be described, these rollers serve to limit movement of the carriage with respect to the pipeline.

As will be more fully described to follow, the cutter heads are mounted on the carriage for movement outwardly from operative trenching position to the position shown in FIG. 3, wherein they may be moved vertically past opposite sides of the pipeline. Also, the lower wheel 24 may be moved from its position engaging the bottom of the pipe, as best shown in FIG. 1, to a position to the side of the pipe, as best shown in FIG. 3, and the two sets of rollers may be released and removed from positions about the pipeline, whereby the wheels and rollers may also be moved vertically past the side of the pipe. In this manner, and upon formation of a starter hole beneath the pipeline to receive the heads and lower wheel, the carriage may be moved downwardly onto supported position on the pipeline. Then, as will be described to follow, the lower wheel may be moved into engagement with the bottom of the pipeline, the rollers connected thereabout, and the cutter wheels moved inwardly to the position best shown in FIGS. 2 and 4, wherein they are at least partially beneath the pipe. Obviously, the apparatus may be removed from the pipeline by a reverse manipulation of the cutter heads and lower wheels.

The axes of the cutter heads are forwardly inclined at a relatively small angle to the vertical so as to dispose their rear sides substantially vertical i.e., perpendicular to the ground surface G. The axes are also outwardly inclined at a relatively angle to small angle vertical so as to dispose their inner sides substantially vertical and close to one another. As will be described to follow, the heads swing about vertical axes so that the eccentricity of their upper and lower ends, as illustrated by the circular broken lines in FIGS. 2 and 3, is maintained as they move between alternate positions.

As best shown in FIG. 1, with the axis of each cutter head inclined forwardly with respect to the vertical, the bottom ends of the cutter heads are upwardly inclined somewhat with respect to the bottom of the trench T. This disposition of the rear sides and bottom ends of the cutter heads has been found especially useful in facilitating removal of cuttings from within cutter head cages.

Each cutter head is supported from the carriage 21 and moved between its inner and outer positions by means of an arm 26 rotatable about a vertical pin 27 mounted on a side of the carriage. Thus, each arm comprises a vertical, angular plate 28 having a sleeve 29 on its rearward end for pivoting about the pin 27, and inner and outer flanges 30 and 31 at its upper and lower edges, respectively. Each cutter head' is rotatably mounted on the outer side of the plate 28 of each arm by means of a hollow shaft 30A extending into the cutter head cage and suspended from a box 32 containing a chain driven by a suitable motor 33 powered by hydraulic fluid from a remote source to be described. The box and thus the shaft 30A and motor 33 are connected to the outer side of plate 28 by a bracket 31A.

Each arm is swung between positions moving the cutter heads close together and more widely spaced apart by means of reciprocating hydraulic actuators 34 and 35. Thus, asshown in FIGS. 2 and 3, the front laterally enlarged end of the carriage 21 includes an I- beam 36, and the actuators connect the forward ends of the arms on each side of the carriage to the ends of the beams 36 on opposite sides thereof. Thus, upon retraction of the piston rods thereof, as shown in FIG. 2, the cutter heads are moved inwardly to the position of FIG. 2, and, upon extension of such rods, as shown in FIG. 3, the cutter heads are moved outwardly to positions beyond opposite sides of the pipeline.

The hollow shaft 30A on each cutter head arm provides a through conduit to receive cuttings from within the cage, and a suction pump 37 is connected to the upper end of the conduit above the motor 32. Thus, when the pump is actuated, cuttings within the cage are drawn upwardly through the hollow shaft A and deposited out the upper end of the side outlet 38 of the pump to one side of the trench T.

Another conduit 39 is also supported on each arm 26 for removing cuttings from the lower rear side of each cutter head. Thus, as shown in the drawings, the conduit 39 is supported on arm plate 28 by means of a bracket 40, and lower flange 31 through which it extends so as to dispose its lower open end adjacent the lower rear side of the cutter head on the arm. A suction pump 41 is connected to the upper end of conduit 39 and has a side outlet 42 for depositing cuttings drawn upwardly through it to the side of the trench. Each of the pumps 37 and 42 may be hydraulically operated by the same source used for operating motors 33.

Channels 43 extend forwardly from their laterally extending I-beam 36 of the carriage 21 to mount a fender 44 across the front end of the carriage. A cylindrically shaped buoyancy tank 48 is supported generally above the fender 44 by means of upright arms 49 pinned to the channels 43. As best shown in FIG. 8, the tank 48 is disposed centrally side-for-side of the carriage.

Also, plates are mounted on the upper opposite sides of the carriage 21 toward its rear end to receive a laterally extending rod 46, which is braced by means of plates 47. A pair of spherical buoyancy tanks 50 are mounted side-by-side above the carriage 21 by means of upright arms 51 connecting them to the rod 46. More particularly, and as shown in FIG. 9, the spherical tanks are of equal size and spaced equally on opposite sides of the center line of the carriage.

The upper end of the tank 48 is braced by means of a rod 52 connected between the top of tank 48 and the upper end ofa post 54 supported on and extending upwardly from the carriage 21, as best shown in FIG. 4. The spherical tanks 50 are braced by means of a rod 53 connected between the upper end of the post 54 and the laterally extending rod 55. As indicated in FIGS. 8 and 9, each tank may be easily disconnected from the rods and the carriage so as to permit their replacement and/or repair. It will also be understood that the tanks may be filled to any desired extent with a fluid of desired weight so as to adjust the buoyancy of the apparatus.

With the cutter heads 25 disposed forwardly of the support wheels 22 and 23, and further with the motor 32 and pumps 37 and 41 located as best shown in FIG. 1, the buoyancy tank 48 will normally have greater capacity than the two spherical tanks 50. Consequently, the center of buoyancy of the apparatus will be generally forwardly and above its center of gravity.

As previously described, regardless of location, the buoyancy tanks lessen the effective weight of the apparatus, and thus facilitate its use with smaller and thus more flexible pipelines. Also, inasmuch as these tanks cause the center of buoyancy of the apparatus to be above its center of gravity, the apparatus is rendered more stable side-by-side. That is, they provide upwardly directed forces which act over relatively large lateral moment arms in resisting any tendency of the carriage and thus the over-all apparatus to tilt to one side or the other. Thus, it is possible to maintain the apparatus in an upright, stable position without any rigid connections between it and the vessel at the water surface. Additionally, there is no need for other lateral stabilizing elements, such as outriggers, slidable over the ground surface G on opposite sides ofthe trench.

The front upper wheel 22 is mounted on the carriage 21 by means of an arm 60, and the rear upper wheel 23 is mounted thereon by means of an arm 61. More particularly, and as best shown in FIG. 7, a bracket 62 secured to one side of the carriage 21 intermediate the wheels has a depending portion to which the arm 60 is pivoted by means of a pin 63 and to which the arm 61 is pivoted by means of a pin 64. Thus, each of the upper wheels is swingable about a horizontal axis transverse to the pipeline.

The wheel 22 is urged downwardly against the pipeline by means of a reciprocating hydraulic actuator 65 connected at its upper end to an upstanding bracket 66 on the carriage 21 and at its lower end to an intermediate point on the arm 60. The wheel 23, on the other hand, is urged downwardly against the top of the pipeline by means'of a reciprocating hydraulic actuator 67 connected at its upper end to an upstanding bracket 68 on the carriage 21 and at its lower end to a midpoint of the arm 61. Thus, upon introduction of hydraulic fluid to the piston end of each actuator, the wheels 22 and 23 are urged downwardly againstthe top of the pipeline with a predetermined force.

As shown in FIGS. 6 and 7, the wheel 22 includes a motor 69 and the wheel 23 includes a motor 70, each motor being reversible so that, in normal operation, the wheels may be rotated in a direction to move the carriage forwardly over the pipeline, and, in certain circumstances, to be rotated in an opposite direction to move the carriage rearwardly. The swinging arms 60 and 61 are connected-directly to the sides of the motors. The actuators and motors are driven from a suitable source of hydraulic fluid.

As also shown in FIGS. 6 and 7, a stop member 71 is secured to the carriage 21 for limiting upward movement of the wheel 22, and a stop member 72 is secured to the carraige to limit upward movement of the wheel 23. As shown in FIG. 7, these stop members have arcuate surfaces for engaging the motor on each wheel when the wheel is moved to its upper limit.

The lower wheel 24 is connected to carriage 21 by means of a pair of arms 73 and 74. Thus, the arm 73 is pivotally connected to the frame by means of a bearing sleeve at its inner end which rotates about a pin 75 held by brackets 76 on the side of the carriage. The opposite end of the arm 73 is in turn pivotally connected to the arm 74 by means of a pin 77, and the free end of the arm 74 is fixedly connected to a bearing 78 for the shaft of the-lower wheel.

The arm 73 is caused to swing about an axis parallel to the pipeline by means of an hydraulic actuator 79 connected at one end to a laterally extending bearing 80 on the opposite side of the carriage 21 and at its opposite end to a plate 81 extending upwardly from the upper end of the arm 73. In this manner, the arm 73, and thus the arm 74 and the lower wheel 24 can be swung from the relatively vertical position of FIGS. 1 and 6 to the relatively horizontal position of FIG. 3.

A hydraulic actuator 82 is connected at one end to a plate extending from the bearing sleeve and at its opposite end to the arm 74 intermediate the pin 77 and the wheel 24. Thus, the actuator 82 can be expanded and contracted so as to swing the arm 74 and thus the wheel 24 about an axis perpendicular to the axis about which the arm 73 is swung.

Thus, when the wheel 24 is to be moved from the position of FIG. 3 to the position of FIG. 1, hydraulic fluid from a suitable source is directed to the actuator 79 so as to extend same and thus move the wheel 24 to a position beneath the pipeline P. Hydraulic fluid is then directed to the actuator 82 to retract same and swing the arms 73 and 74, and thus the wheel 24, upwardly to a position engaging the bottom of the pipeline, as shown in FIG. 3. Obviously, a reversal of this operation will cause the wheel 24 to be first swung downwardly to a position beneath the pipeline, and then raised upwardly to one side of the pipeline.

The yieldable force due to hydraulic fluid within the actuator 82 will not only raise the wheel 24 into engagement with the bottom of the pipeline, but will also hold the wheel against the bottom of the pipeline with a desired force, and thus cause the wheels 22 and 23 to move against the stop members 71 and 72. Consequently, the wheel 24 serves to tightly engage the pipeline between it and the upper wheels 22 and 23, and thus control the traction between the driven wheels 22 and 23 and the pipeline. 7 As previously described, there are two sets 83 of rollers, one of which is disposed just forwardly of the front wheel 22 and the other of which is disposed just rearwardly of the rear top wheel 23. As shown in the drawings, each set of rollers is carried within a frame adapted to open during installation of the carriage on the pipeline as well as lifting of the carriage from the pipeline, but normally adapted to be latched in position to dispose the rollers about all four sides of the pipeline.

As best shown in FIG. 5, the frame of each set of rollers 83 includes an upper part 84 fixed to the carriage 21 and a lower part 85 pivotally connected to the upper part by means of a pin 86. A hydraulic actuator 87 is connected between the upper frame part 84 and the lower part 85 for swinging the lower part between the closed position of FIG. and an open position (not shown), wherein the lower part 85 is to one side of the pipeline P. When the lower frame member is in its struction so as to absorb dynamic loading in the event the front end of the apparatus engages relatively immovable obstructions.

The upper roller of each set lies relatively close to, and in fact may touch, the top of the pipeline P when the wheels 22 and 23 are in their uppermost positions. On the other hand, the lower and side rollers of each set are spaced slightly from pipeline P.

As previously described, the rollers limit movement of the apparatus with respect to the pipeline in the event of jarring or vibration which might untrack one ormore of the wheels 22, 23 and 24 from the pipeline, or possibly a loss of power fluid in connection with the actuators holding any one or more of the wheels 22, 23 and 24 in engagement with the pipeline. In fact, the rollers would limit movement of the frame with respect to the pipeline even if all power for operating the apparatus was lost, because the frame is mechanically locked about the pipeline.

As well known in this art, underwater pipelines are frequently covered with a cement coating, which may not be continuous along the pipeline. For example, in

many pipelines, a coating is not applied to the joints bottom of the pipeline, the force due to the actuator 82- merely causes the lower wheel 24 to move upwardly into the area from which the coating had been removed, thereby maintaining tight engagement with the pipeline and thus between the pipeline and the driving wheels 22 and 23. On the other hand, since a constant load is also maintained on each of the wheels 22 and 23, each of them will also move inwardly into a portion of the pipeline from which the coating has been removed, and then continue to move out of such portion and back onto the undisturbed, coated portion of the pipeline. Thus, both wheels 22 and 23 are always in driving engagement with some portion of the pipeline.

The two sets of rollers are also .helpful in maintaining the desired traction between the wheels and the pipeline. For example, even if the top roller of the front set 83 is engaged with a cement coating of the pipeline, while the'wheel 22 is depressed into an uncoated portion of the pipeline, the upper roller of the front set will prevent the carriage from tilting forwardly because, as previously described, and as shown in FIG. 5, it is close to if not engaged with the top of the pipeline. The same is true, of course, of the possible movement of the top wheel 23 into an uncoated portion of pipe, while the top roller of the rear set of rollers is engaged with the coating portion.

The wheels 22 and 23 are preferably spaced apart a distance greater than the maximum length of uncoated joints along the pipeline, so that both top rollers would normally not be within an uncoated portion of the pipeline. On the other hand, if only one of the top rollers is in an uncoated portion of the pipe, the upwardly directed force due to bottom wheel 24 will maintain the carriage in a level position longitudinally with respect to the pipeline since it directs a force upwardly about the axis of the top roller of the other set of rollers.

As previously mentioned, hydraulic fluid may be supplied to the various hydraulic actuators of the apparatus from a remote source, such as a vessel at water level. Thus, for example, flexible hoses (not shown) from the vessel would connect with a header or distribution conduit on the carriage such as shown at 91,

2. Apparatus of the character defined in claim 1, in-

,cluding a reciprocable actuator means connected to the arms for moving the cutter heads between said positions.

3. Apparatus of the character defined in claim 1, I

- other conduits for depositing cuttings to a position and other hoses (not shown) would connect the distribution conduit 91 to individual actuators and motors for operating them in a well known manner. Controls 7 for operating such actuators and motors in desired Additional jets 94 are provided for removing soil which might become stuck to the bottom end of a cutter head 25 or in the angular space between the bottom end of the cutter head and the trench T. Each jet may include a manifold depending from a distribution conduit connected to the carriage, as shown in FIG. 4, and fluid under pressure is supplied to each jet from a reservoir enclosed within the elongate portion of the carriage 21, as shown, for example, in FIG. 4, with hoses (not shown) connecting each such distribution conduit with the reservoir and a pump (not shown) being provided for maintaining the jetting fluid under pressure.

From the foregoing it can be seen that this invention is one well adapted to attain all of the ends and objects hereinabove set forth, together with other advantages which are obvious and which are inherent to the apparatus.

It will be understood that certain features and subcombinations are of utility and may be employed without reference to other features and subcombinations. This is contemplated by and is within the scope of the claims.

As many possible embodiments may be made of the invention without departing from the scope thereof, it is to be understood that all matter herein set forth or shown in the accompanying drawings is to be interpreted as illustrative and not in a-limiting' sense.

The invention having been described, what is claimed is: t

1. Apparatus for digging a trench beneath a pipeline, comprising a carriage supportable on the pipeline for forward movement longitudinally therealong, a pair of arms pivotally connected to the carriage about a substantially vertical axis at least one cutter head rotatably supported by each of the arms about a generally vertical axis, means mounted on each arm for rotating the cutter head thereon, a conduit mounted on each arm to receive cuttings from the cutter head on said arm, and a suction pump connected to each conduit for depositing said cuttings to a position removed from the trench beneath the pipeline.

removed from the trench.

5. Apparatus of the character defined in claim 1, wherein eachcutter comprises a cage rotatable about a substantially vertical axis, and the conduit is behind the cage to receive cuttings removed thereby.

6. Apparatus of the character defined in claim 1, wherein the axis about which each arm is swung is disposed rearwardly of said cutter head, and a wheel is mounted on the carriage for rolling along the pipeline forwardly of said axes.

7. Apparatus for digging a trench beneath a pipeline, comprising a carriage, a wheel assembly supporting the carriage for movement longitudinally forwardly along the pipeline, cutter means rotatable about a substantially vertical axis, means for supporting the cutter means from the carriage for movement therewith forwardly of said wheel assembly, a buoyancy tank mounted on the carriage with its center of buoyancy forwardly of said cutter means and wheel assembly, and

at least one other buoyancy tank mounted on the carriage with its center of buoyancy rearwardly of the wheel assembly.

8. Apparatus for digging a trench beneath a pipeline, comprising a carriage, a wheel assembly supporting the carriage for movement longitudinally forwardly along the pipeline, cutter means rotatable about a substantially vertical axis, and means for supporting the cutter means from the carriage for movement therewith forwardly of said wheel assembly, wherein. the cutter means comprises a pair of rotatable cutter heads each connected to the carriage for swinging about a substantially vertical axis between positions in which they are at least partially beneath the pipeline and in which they are relatively widely spaced apart for movement vertically past opposite side of the pipeline.

9. Apparatus of the character defined in claim 8, wherein each cutter head is forwardly of the axis about which it swings, and said wheel assembly includes a wheel engaging the top of the pipeline intermediate each said cutter head and axis.

10. Apparatus for digging a trench beneath a pipeline, comprising a carriage, first and second wheels mounted on the carriage for engaging the top and bottom of the pipeline, respectively, means for causing said first and second wheels to engage the top and bottom of the pipeline with a constant load, means for driving at least one of said wheels so as to move the carriage along the pipeline, cutter means, and means supporting the cutter means from the carriage for movement therewith at least partially beneath the pipeline.

11. Apparatus of the character defined in claim 10, including at least one buoyancy tank mounted on the carriage for movement therewith.

12. Apparatus of the character defined in claim 10, including means connecting the second wheel to the carriage for movement between said position engaging the bottom of the pipeline and another position to permit said carriage to be moved to and from a position in which said first wheel is supported on the pipeline.

13. Apparatus of the character defined in claim 12, wherein the second wheel connecting means comprises a first arm pivotally connected to the carriage for swinging laterally about a horizontal axis generally parallel to the pipeline, a second arm pivotally connected to the first arm for swinging about an axis perpendicular to said horizontal axis, and means for selectively causing 'each of said arms to swing about its respective axis.

14. Apparatus of the character defined in claim 10, including rollers carried by the carriage for disposal at least close to the top, bottom and opposite sides of the pipeline forwardly and rearwardly of said wheels.

15. Apparatus for digging a trench beneath a pipeline, comprising a carriage, a pair of wheels mounted on the carriage for engaging the top of the pipeline at longitudinally spaced apart locations therealong, a third wheel mounted on the carriage for engaging the bottom of the pipeline intermediate the pairv of wheels, means for causing the wheels to engage said pipeline with a constant load, means for driving at least one of said wheels so as to move the carriage along the pipeline, cutter means, and means supporting the cutter means from the carriage for movement therewith.

16. Apparatus of the character defined in claim 15, including at least one buoyancy tank mounted on the carriage for movement therewith.

17. Apparatus of the character defined in claim 15, including rollers supported on the carriage for disposal at least close to the top, bottom and opposite sides of the pipeline forwardly and rearwardly of said pair of wheels.

18. Apparatus for digging a trench beneath a -pipeline, comprising a carriage, a pair of wheels mounted on the carriage for engaging the top of the pipeline at longitudinally spaced apart locations therealong, means urging said pair of wheels against the pipeline with a constant load, means for limiting upward movement of said pair of wheels with respect to the carriage, a third wheel mounted on the carriage for engaging the lower side of the pipeline intermediate the pair of wheels, means for driving said pair of wheels so as to move the carriage along the pipeline, cutter means, and means supporting the cutter means from the carriage for movement therewith.

19. Apparatus of the character defined in claim 18, including at least one buoyancy tank mounted on the carriage for movement therewith.

20. Apparatus of the character defined in claim 18, including rollers supported on the carriage for disposal at least close to the top, bottom and opposite sides of the pipeline forwardly and rearwardly of said wheels.

21. Apparatus for digging a trench beneath a pipeline, comprising a carriage supportable on the pipeline for movement longitudinally therealong, cutter means, means supporting the cutter means from the carriage for movement therewith at least partially beneath the pipeline, a wheel assembly supported by the carriage or rolling engagement W] h the pipeline,

means for driving at least a portion of said wheel assembly so as to move the carriage along the pipeline, a buoyancy tank mounted'on the carriage near the forward end of the carriage and arranged with its center of buoyancy generally above the axis of the pipeline, a pair of buoyancy tanks mounted on the carriage above the pipeline near its rearward end and arranged in laterally spaced relation with their centers of buoyancy at generally equal lateral distances from the axis of the pipeline.

22. Apparatus of the character defined in claim 21 wherein said wheel assembly is positioned intermediate the tanks nearthe forward and rear ends of the carriage.

23. Apparatus of the character defined in claim 21, wherein said cutter means is intermediate said tanks near the forward and rear ends of the carriage.

24. Apparatus of the character defined in claim 22, wherein said cutter means is intermediate said tanks near the forward and rear ends of the carriage. 

1. Apparatus for digging a trench beneath a pipeline, comprising a carriage supportable on the pipeline for forward movement longitudinally therealong, a pair of arms pivotally connected to the carriage about a substantially vertical axis at least one cutter head rotatably supported by each of the arms about a generally vertical axis, means mounted on each arm for rotating the cutter head thereon, a conduit mounted on each arm to receive cuttings from the cutter head on said arm, and a suction pump connected to each conduit for depositing said cuttings to a position removed from the trench beneath the pipeline.
 1. Apparatus for digging a trench beneath a pipeline, comprising a carriage supportable on the pipeline for forward movement longitudinally therealong, a pair of arms pivotally connected to the carriage about a substantially vertical axis at least one cutter head rotatably supported by each of the arms about a generally vertical axis, means mounted on each arm for rotating the cutter head thereon, a conduit mounted on each arm to receive cuttings from the cutter head on said arm, and a suction pump connected to each conduit for depositing said cuttings to a position removed from the trench beneath the pipeline.
 2. Apparatus of the character defined in claim 1, including a reciprocable actuator means connected to the arms for moving the cutter heads between said positions.
 3. Apparatus of the character defined in claim 1, wherein each cutter comprises a cage rotatable about a substantially vertical axis, and the conduit extends within the cage.
 4. Apparatus of the character defined in claim 3, including another conduit mounted on each arm behind the cage, and a suction pump connected to each of said other conduits for depositing cuttings to a position removed from the trench.
 5. Apparatus of the character defined in cLaim 1, wherein each cutter comprises a cage rotatable about a substantially vertical axis, and the conduit is behind the cage to receive cuttings removed thereby.
 6. Apparatus of the character defined in claim 1, wherein the axis about which each arm is swung is disposed rearwardly of said cutter head, and a wheel is mounted on the carriage for rolling along the pipeline forwardly of said axes.
 7. Apparatus for digging a trench beneath a pipeline, comprising a carriage, a wheel assembly supporting the carriage for movement longitudinally forwardly along the pipeline, cutter means rotatable about a substantially vertical axis, means for supporting the cutter means from the carriage for movement therewith forwardly of said wheel assembly, a buoyancy tank mounted on the carriage with its center of buoyancy forwardly of said cutter means and wheel assembly, and at least one other buoyancy tank mounted on the carriage with its center of buoyancy rearwardly of the wheel assembly.
 8. Apparatus for digging a trench beneath a pipeline, comprising a carriage, a wheel assembly supporting the carriage for movement longitudinally forwardly along the pipeline, cutter means rotatable about a substantially vertical axis, and means for supporting the cutter means from the carriage for movement therewith forwardly of said wheel assembly, wherein the cutter means comprises a pair of rotatable cutter heads each connected to the carriage for swinging about a substantially vertical axis between positions in which they are at least partially beneath the pipeline and in which they are relatively widely spaced apart for movement vertically past opposite side of the pipeline.
 9. Apparatus of the character defined in claim 8, wherein each cutter head is forwardly of the axis about which it swings, and said wheel assembly includes a wheel engaging the top of the pipeline intermediate each said cutter head and axis.
 10. Apparatus for digging a trench beneath a pipeline, comprising a carriage, first and second wheels mounted on the carriage for engaging the top and bottom of the pipeline, respectively, means for causing said first and second wheels to engage the top and bottom of the pipeline with a constant load, means for driving at least one of said wheels so as to move the carriage along the pipeline, cutter means, and means supporting the cutter means from the carriage for movement therewith at least partially beneath the pipeline.
 11. Apparatus of the character defined in claim 10, including at least one buoyancy tank mounted on the carriage for movement therewith.
 12. Apparatus of the character defined in claim 10, including means connecting the second wheel to the carriage for movement between said position engaging the bottom of the pipeline and another position to permit said carriage to be moved to and from a position in which said first wheel is supported on the pipeline.
 13. Apparatus of the character defined in claim 12, wherein the second wheel connecting means comprises a first arm pivotally connected to the carriage for swinging laterally about a horizontal axis generally parallel to the pipeline, a second arm pivotally connected to the first arm for swinging about an axis perpendicular to said horizontal axis, and means for selectively causing each of said arms to swing about its respective axis.
 14. Apparatus of the character defined in claim 10, including rollers carried by the carriage for disposal at least close to the top, bottom and opposite sides of the pipeline forwardly and rearwardly of said wheels.
 15. Apparatus for digging a trench beneath a pipeline, comprising a carriage, a pair of wheels mounted on the carriage for engaging the top of the pipeline at longitudinally spaced apart locations therealong, a third wheel mounted on the carriage for engaging the bottom of the pipeline intermediate the pair of wheels, means for causing the wheels to engage said pipeline with a constant load, means for driving at least one of said wheels so as to move The carriage along the pipeline, cutter means, and means supporting the cutter means from the carriage for movement therewith.
 16. Apparatus of the character defined in claim 15, including at least one buoyancy tank mounted on the carriage for movement therewith.
 17. Apparatus of the character defined in claim 15, including rollers supported on the carriage for disposal at least close to the top, bottom and opposite sides of the pipeline forwardly and rearwardly of said pair of wheels.
 18. Apparatus for digging a trench beneath a pipeline, comprising a carriage, a pair of wheels mounted on the carriage for engaging the top of the pipeline at longitudinally spaced apart locations therealong, means urging said pair of wheels against the pipeline with a constant load, means for limiting upward movement of said pair of wheels with respect to the carriage, a third wheel mounted on the carriage for engaging the lower side of the pipeline intermediate the pair of wheels, means for driving said pair of wheels so as to move the carriage along the pipeline, cutter means, and means supporting the cutter means from the carriage for movement therewith.
 19. Apparatus of the character defined in claim 18, including at least one buoyancy tank mounted on the carriage for movement therewith.
 20. Apparatus of the character defined in claim 18, including rollers supported on the carriage for disposal at least close to the top, bottom and opposite sides of the pipeline forwardly and rearwardly of said wheels.
 21. Apparatus for digging a trench beneath a pipeline, comprising a carriage supportable on the pipeline for movement longitudinally therealong, cutter means, means supporting the cutter means from the carriage for movement therewith at least partially beneath the pipeline, a wheel assembly supported by the carriage for rolling engagement with the pipeline, means for driving at least a portion of said wheel assembly so as to move the carriage along the pipeline, a buoyancy tank mounted on the carriage near the forward end of the carriage and arranged with its center of buoyancy generally above the axis of the pipeline, a pair of buoyancy tanks mounted on the carriage above the pipeline near its rearward end and arranged in laterally spaced relation with their centers of buoyancy at generally equal lateral distances from the axis of the pipeline.
 22. Apparatus of the character defined in claim 21 wherein said wheel assembly is positioned intermediate the tanks near the forward and rear ends of the carriage.
 23. Apparatus of the character defined in claim 21, wherein said cutter means is intermediate said tanks near the forward and rear ends of the carriage. 